Opencl is not working on the Radeon HD 7750/8740 / R7 250E
Your GPU is GCN 1st gen. That's why it's not supported.
Cegaton pointed out in a comment Can&#39;t use Radeon GPU to render in cycles (R9 270)
Which is where I figured that out from this list here https://en.wikipedia.org/wiki/List_of_AMD_graphics_processing_units

EDIT: Originally I claimed and cited that Nvidia's Cuda core programming has higher support and faster speeds. However, Cegaton pointed out in the comments that as of Blender Version 2.79 OpenCL has made significant improvements. While OpenCL still is undergoing active development it appears to beat NVidia in the benchmarks, at least in tests comparing a midrange ~$200 graphics cards